'90 GMC TBI 350 can't cure spark knock...

[rms-man]

Please give some advice. Cylinders 1,3,5 all had about 90psi while the rest had 185 psi. Used a leakdown tester anf neither of those 3 leaked down from 100psi. That doesn't make much sense to me and now I'm thinking maybe my spark plug hose had a valve core in it not allow them to leak down or something. Why would I have such weak cylinders, but no leakdown? I've replaced plugs, wires, cap & rotor. Used super unleaded, checked timing as per the card underhood. I re-checked the new plugs in the 3 bad cylinders after 1 week and they were very gray. All I get is what sounds like spark knock while giving it more than 1/4 throttle. When this all started it got a miss at idle, but seems to drive with normal power. Any ideas what to look for or try next.

 

Thanks,

Joshua

[GM-Tech]

Used a leakdown tester anf neither of those 3 leaked down from 100psi. That doesn't make much sense to me and now I'm thinking maybe my spark plug hose had a valve core in it not allow them to leak down or something.

 

Makes no sense to me either. Never seen an engine that had 0% leakdown. It does sound like you forgot to remove the valve core from the hose's spark plug fitting.

[rms-man]

Yeah I'm going to do the leakdown again on Monday and make sure there isn't a valve core.

 

Joshua

[rms-man]

Yeah I'm going to do the leakdown again on Monday and make sure there isn't a valve core.

 

Joshua

 

<{POST_SNAPBACK}>

 

 

 

Sure enough the valve core was still in the hose I used the first time. Found I had a few cylinders with weak rings, but the worst was a blown head gasket between #3 & #5. Then after pulling it all apart to try and replace just the head gaskets I found that piston #5 had broken the skirts off and the upper ring land was missing a piece the size of a small paper clip. So, with the miles that were on that engine and the rough shape of the heads I just bought a rebuilt long block and started the swap. Now I'm stuck trying to get my hands on the exhaust manifold spreader tools so I can get them to go back on. Other than that I should have a nice engine to last me the life of the truck. Thanks again for your reply GM-Tech.

 

Joshua

[Mr. Smith]

Yeah I'm going to do the leakdown again on Monday and make sure there isn't a valve core.

 

Joshua

 

<{POST_SNAPBACK}>

 

 

 

Sure enough the valve core was still in the hose I used the first time. Found I had a few cylinders with weak rings, but the worst was a blown head gasket between #3 & #5. Then after pulling it all apart to try and replace just the head gaskets I found that piston #5 had broken the skirts off and the upper ring land was missing a piece the size of a small paper clip. So, with the miles that were on that engine and the rough shape of the heads I just bought a rebuilt long block and started the swap. Now I'm stuck trying to get my hands on the exhaust manifold spreader tools so I can get them to go back on. Other than that I should have a nice engine to last me the life of the truck. Thanks again for your reply GM-Tech.

 

Joshua

 

<{POST_SNAPBACK}>

 

 

 

 

http://www.bobstools.net/Store/LS13000.html

 

Thats what you need. Any Lisle dealer ought to be able to get it. Napa as well.